Saving changes is not permitted. The changes you have made require the following tables to be dropped and re-created. You have either made changes to a table that can't be re-created or enabled the option Prevent saving changes that require the table to be re-created.
It is so self exlpanatory still a lot of people have problems to solve it.
You can follow the steps bellow to fix this:
Select "Options" from the Tools menu:
Then Uncheck the "Prevent saving changes that require table re-creation" option and click OK:
